About Commit Protocols and Recoverability

The safe, remote, and region commit protocols coordinate messages to the journal with transaction commits. If a Storage Manager (SM) process terminates unexpectedly, journaling messages are used to ensure that the archive is in a consistent state when it rejoins the database.

Journaling is always enabled (for data durability). For the purpose of redundancy, at least two SMs should be used.
For performance and latency purposes, NuoDB recommends that the journal and the archive are located on different disks. NuoDB recommends also recommends using different disk controllers. If either the journal or the archive is unavailable, or one of them is corrupt, then an SM cannot be started.

Replication files that are required for journaling are stored in a specified journal directory; if a directory is not specified, the default directory is journal in the archive directory. For information on using --journal-dir and other journaling-related options, see Database Options.